1. Field
The field of the invention is guides and guards for cables directed by sheaves employed in the drilling and servicing of oil and gas wells.
2. State of the Art
In the drilling and servicing of oil and gas wells, apparatus is often lowered and raised by cables operating within boreholes extending deeply into the earth. Cables typically run from a winch through one or more sheaves changing the direction of the cable, ultimately directing it downwardly into the borehole.
Two principal problems, both addressed by the present invention, are associated with deployment and retrieval of cable in the boreholes. The cables run in the vicinity of workmen, whose hands or other body appendages are too often seriously injured when caught between the moving cables and rotating sheave wheels. In other cases the cable is drawn out of the cable guiding groove provided about the circumference of the sheave wheel. This "jumping of the sheave" is very dangerous to operating personnel, and is destructive to the sheave structure. The running cable may quickly abrade through the sheave structure, releasing the cable to whip violently and dangerously out of control. The cable is in danger of jumping the sheave when it can approach the sheave not directly, but only at a substantial angle to the plane of the sheave wheel called a "fleet angle".
One common operation is the retrieval of data from deep within boreholes. An instrument("sonde") is connected to an armored cable containing electrical conductors. The data measured by the sonde is sent up the conductors to the surface to be recorded and analyzed. The conductive cable is called a "wireline" in the industry, and the measurement of data from the borehole is called "wireline operations". In a typical wireline operation, the cable is deployed from a winch cable reel through a first rigging sheave located on the drilling rig floor. This sheave is called the floor sheave, and the line goes upward from it to a second sheave suspended from a block on or near the center of the top of the drilling derrick. This sheave is called the top sheave, and the line descends from it downwardly into the borehole.
Safety hazard in this instance occurs at the floor sheave, which the cable runs through at about knee height. Drilling rig floors typically have poor and uncertain footing, being covered with oil, drilling mud, water and other well fluids, and frequently ice. A slipping workman reflexively reaches for support, often grasping the moving cable, which carries his hand into the sheave to be pinched between the running line and the rotating sheave wheel. Hands, arms, legs or clothing may be also ensnared and carried by the moving line into its confluence with the sheave wheel.
On offshore oil platforms, the sheaves used for routine drilling and well service operations are frequently attached to the deck of the platform. Lines of differing sizes are used for various necessary functions in proximity to workmen. The number of sizes of running lines enhances the danger of limb and clothing entrapment.
Sizeable fleet angles are more likely to develop on sheaves fastened to decks or bulkheads rather than on those suspended overhead on swivels. Although the swivel mount in theory causes the sheave wheel to become aligned with the plane of the approaching and departing cable, sizeable fleet angles are in practice still developed because of the complications of friction, and other imperfections in the swiveling process. The fleet angle problem, then, occurs both with swivel suspended sheaves as well as with floor or bulkhead mounted sheaves.
The prior art discloses attempts to block the entry of hands or clothing from entering the sheave assembly to be pinched between the cable and the wheel, and to guide the cable sufficiently to prevent cable jump from the sheave. U.S. Pat. Nos. 775,118 and 1,242,656 each disclose a pair of cable guiding arms pivoted from sheave wheel mounting bodies or housings. Each pair of arms at its end distant from the housing joins with a fixed integral guiding loop. The cable engaging loop, being integral with the pivoted arm, restricts the usable angle of the cable guide. This can only be countered by enlarged guiding loop size, impairing its guiding and guarding performance. Accordingly, these cable guides are inherently useful only within limited ranges of cable position and angle of approach to the sheave wheel. In the disclosed embodiments, this limitation is countered by providing guide arms of excessive length, less effectively guarding against digit or limb entrapment between the sheave and the cable. When enlarged guiding loop sizes are employed, the guiding is so impaired that cable jump may often still be a danger.
The fixed integral relationship between the guiding loops and pivoting arms, the length of the arms, the point of pivotal attachment to the housing, and the play allowed the cable within the loops, all must be selected to provide the best combination. The guiding and guarding performance is necessarily compromised to achieve a balance between these competing factors. The same disadvantage is also disclosed in U.S. Pat. No. 1,379,868, wherein the arms are brackets fixedly secured to the housing, from which cable guide sleeves are pivoted. This allows for no adjustment of position of the guide sleeves during operation. Some lateral guiding flexibility is provided by the use of very loose pivotal connections. No automatic adjustment of guide sleeve position is possible. The brackets must be positioned to accommodate specific angles of approach of the cable to the sheave assembly.
U.S. Pat. No. 1,365,951 discloses a combination of a fixed cord guide and a pivoted arm carrying such a guide. Again, the guiding element is fixedly secured to the arm. Similar disclosures are found in U.S. Pat. Nos. 346,084, 349,520 and 126,391. The latter two are adapted for chain sheaves, but nevertheless disclose pivoting arms with fixed guide elements at each end.
None of the devices disclose embodiments capable of sufficiently versatile application to provide efficient, dependable guiding and guarding of oil and gas well wireline cables.